The invention relates to a circuit system for the generation of a direct control voltage which is dependent upon an alternating voltage and composed of a charging circuit which can be charged through one circuit path and discharged through another circuit path.
The use of such a circuit system as a control voltage generator in a compander system has been previously disclosed in the German patent specification No. 24 06 258 and counterpart U.S. Pat. No. 3,969,680.
Into the input of the control voltage generator is fed, on compression, the alternating voltage output signal and, on expansion, the alternating voltage input signal from an amplifier with electronically controllable amplification located in the useful signal path is fed to the control voltage generator input via another electronically controllable amplifier. The direct voltage output signal from the control voltage generator is fed both into the control input of the amplifier located in the useful signal path and into the control input of the other amplifier. In this context the action of the control voltage generator is such that when a threshold value is exceeded, the generator produces a steeply rising direct voltage through the alternating voltage fed into the input of the control voltage generator. This steeply rising direct voltage varies the amplification of the amplifier in the useful signal path until the alternating current at the input of the control voltage generator has dropped back again below the aforesaid threshold value.
A rapid variation of the transmission factor of the amplifiers located in the signal path is of importance specifically when the level of the useful signals varies in jumps over large ranges. Otherwise there exists, in the case of a jump in level from a small to a large value, the risk of overshooting and thereby overdriving of the transmission channel. Noise signals would become audible during the transition time of re-adjustment in cases of jumps in levels in the reverse direction.
The control voltage generator for the control of the transmission factor of the amplifiers located in the useful signal path must therefore furnish a direct control voltage which is rapidly adapted to the momentary level of the useful signal. A condition for this is a short charging or discharging time constant effective on the occurrence of large jumps in levels at the charging capacitor of the control voltage generator. Such a short time constant can be achieved through appropriate rating of the charging or discharging current source.
In the stationary state or during slow level variations the charging or discharging time constant must however be large so that the direct control voltage does not vary in the rhythm of the useful signal frequency. Such a behavior would produce a large non-linear distortion factor.
The result of these considerations is that two contradictory requirements have to be met when fixing the charging and discharging time constants for the control voltage generator. One compromise for a solution is the variation of the discharging and charging time constants as a function of the jumps in level of the useful signal.
The adaptation of the charging and discharging time constants can be achieved through the duty factor, or duration of activation, of a switch located in the charging or discharging current path. The duty factor could in this context be rated in accordance with the time during which a useful signal exceeds in the comparator the limits preset by reference voltages. This time represents at small excursions, an accurate measure for the extent of the excursion, i.e. the ratio between the useful signal amplitude and the reference voltage. This time increases however only little at large excursions so that any assignment of duration of excursion and extent of excursion becomes inaccurate. This approach is technically difficult to master.